The present invention relates generally to information transfer in a communication network, and specifically to a protocol for transmission over a passive optical network.
A point-to-multi-point passive optical network (PON) operates as a communication system by broadcasting optical signals downstream from a central unit, herein termed an optical line termination (OLT), to optical network terminations (ONTs). The signals are transferred from the OLT to the ONTs via fibre optic cables and passive optical splitters, which comprise the physical fabric of the network. For upstream communication, each ONT must be able to transmit signals which are not interfered with by other ONTs. One of the methods known in the art for performing such upstream and downstream transmissions is by using time domain multiple access (TDMA), wherein each ONT is allocated a window when only it can transmit, and where the OLT also has windows for transmission to specific ONTs. Other methods for avoiding interference include transmitting signals at different wavelengths, using wavelength domain multiple access (WDMA). Combinations of TDMA and WDMA are also known in the art.
U.S. Pat. No. 5,173,899 to Ballance, whose disclosure is incorporated herein by reference, describes a method for communication in a passive optical network. An OLT transmits downstream TDM frames, the frames including information, a synchronization signal, and grants (to transmit upstream) to downstream ONTs. The ONTs transmit upstream TDM signals responsive to the grants and the synchronization signal.
U.S. Pat. No. 5,355,368 to Dore et al., whose disclosure is incorporated herein by reference, describes a method for allocating timeslots in a TDMA point-to-multi-point network. The network operates in a half-duplex manner, i.e., terminals of the network alternate between sending and receiving. The method reduces the xe2x80x9cdeadxe2x80x9d time needed between adjacent downstream timeslots of an OLT, the dead time being the round-trip time of transmission between the OLT and an ONT in the network. The reduction is implemented by giving a first ONT an authorization to send while it is receiving information addressed to a second ONT.
U.S. Pat. No. 5,515,379 to Crisler et al., whose disclosure is incorporated herein by reference, describes a system for timeslot allocation within a communication system. A communication unit transmits a first packet of data requesting permission to transmit to a timeslot allocator. The packet contains either a request for allocation of a number of timeslots, or a request to transmit multiple packets of data. In either case, the allocator allocates contiguous time slots to the unit which the unit uses for transmission of its packets.
U.S. Pat. No. 5,528,592 to Schibler et al., whose disclosure is incorporated herein by reference, describes a method for route processing asynchronous transfer mode (ATM) cells. (A packet is comprised of a plurality of ATM cells.) The method consists of receiving, in a route cell buffer, cells corresponding to the beginning and end of a packet. A router determines routing information for the packet from these cells. The information includes a routing label determining an output port for the packet, and an identifier that determines switching paths connecting a packet source to a destination of the packet.
U.S. Pat. No. 5,838,687 to Ramfelt, whose disclosure is incorporated herein by reference, describes a slot reuse method in a Dynamic Synchronous Transfer Mode (DTM) segmented network. Access to slots is controlled by slot tokens, and writing to a slot may only be performed by a controller owning the corresponding token for that slot. A block token is used to represent a group of tokens in a single control message. The method consists of extending the DTM block token format to include parameters describing segments between source and destination nodes. Block token capacity is reserved only on segments between the nodes, and enables simultaneous transmissions in the same slot over disjointed segments of the network.
U.S. Pat. No. 5,982,780 to Bohm et al., whose disclosure is incorporated herein by reference, describes centralized and distributed management of communication resources in a DTM network. In the centralized version a server node is assigned tokens corresponding to time slots for unidirectional data flow on a communications link. The server, if it has available capacity, reserves and transfers tokens to other nodes on the link, according to requests from those nodes. In the distributed version the function of the server is spread amongst two or more nodes connected to the link.
It is an object of some aspects of the present invention to provide a method for transmitting signals in a communications network.
It is a further object of some aspects of the present invention to provide a method for transmitting TDM signals in a passive optical network (PON).
In a preferred embodiment of the present invention, an optical line termination (OLT) transmits optical signals downstream to a plurality of optical network terminations (ONTs). The OLT is coupled via a passive optical distribution fabric to the ONTs, so forming a PON. The OLT acts as a controller of the downstream signals, and also of signals sent upstream by the ONTs and received by the OLT. The downstream signals are sent in the form of frames having a constant period, and each downstream frame comprises a number of xe2x80x9ctimeslots.xe2x80x9d Each timeslot is a fixed number of bytes, and within each frame the OLT allocates timeslots directed to each of the ONTs in a dynamic manner. Timeslot allocation for each frame is implemented according to quantities of data to be transmitted to/from each ONT, as determined by the OLT. Most preferably, the allocation of timeslots for each downstream frame is performed at substantially the same rate as the frames are transmitted. By allocating varying numbers of timeslots to each downstream frame or to an upstream window (explained below), the OLT effectively configures each frame or window in a variable TDM manner, with variable length times for each ONT which receives data.
Upstream signals are transmitted in a TDM manner from individual ONTs in windows, the windows being transmitted according to time pointers allocated by the OLT.
Both upstream and downstream signals comprise data transmitted according to one or more services, which may individually operate according to completely different protocols. Services typically include constant bit rate services and packet based services. Preferred embodiments of the present invention transfer data regardless of the type of service the data is transmitted under.
Upstream and downstream signals are transferred between the OLT and the ONTs via channels which are mapped as a one-to-one mapping from the services. The channels and channel parameters, such as bandwidth, are allocated by an operator of the PON, either at initialization or during operation of the PON. Each channel uses timeslots which are allocated according to bandwidth requirements of the channel""s service.
Data for a specific channel in a downstream frame may be distributed within the frame non-contiguously. Also, data for a specific channel in an upstream window may be distributed within the window non-contiguously. Enabling a specific channel in upstream windows and downstream frames to be arranged non-contiguously significantly enhances the flexibility and efficiency of transmission of these signals, compared to systems which do not allow non-contiguous transmission.
There is therefore provided, according to a preferred embodiment of the present invention, a method for downstream communication from a central transmission point to a plurality of receiving end points by time division multiplexing of a sequence of frames, each of which is divided into multiple timeslots, the method including:
receiving at the central transmission point data for transmission to the end points, the data including at least a first quantity of first data for transmission to a first end point among the plurality of the end points and a second quantity of second data for transmission to a second end point among the plurality of the end points, such that the first and second quantities are variable from each of the frames to the next in the sequence;
responsive to the first and second quantities, allocating in each of the frames a first number of the timeslots to carry the first data to the first end point, and a second number of the timeslots to carry the second data to the second end point, such that the first and second numbers are variable from each of the frames to the next in the sequence responsive to variations in the first and second quantities of the data; and
transmitting the data from the central transmission point to the end points during the allocated timeslots.
Preferably, the central transmission point includes an optical line terminal (OLT), and the end points include optical network terminals (ONTs), wherein the OLT and ONTs are operative as transceivers in a passive optical network.
Preferably, the data includes sets of data which are conveyed via respective different industry-standard services.
Preferably, the plurality of end points includes sets of end points operative at different wavelength groups, and the data includes respective sets of data which are conveyed between the central transmission point and the respective sets of end points via the different wavelength groups.
Preferably, the data includes sets of data which are conveyed via respective different channels, wherein each channel transfers data via a service coupled to the central transmission point and at least one of the end points.
Further preferably, the method includes assigning each channel a respective bandwidth, wherein allocating the first and the second numbers of timeslots includes allocating the first and the second numbers of timeslots responsive to the bandwidth of each of the channels.
Preferably, assigning each channel the respective bandwidth includes changing the respective bandwidth to a different bandwidth responsive to a request received by the central transmission point.
Further preferably, the first and second numbers of timeslots are allocated responsive to respective first and second data parameters stored in a memory comprised in the central transmission point.
Preferably, a total of the first number and the second number of the timeslots is less than or equal to a bandwidth of each of the sequence of frames.
Further preferably, the data includes one or more further quantities of data for transmission to respective one or more further end points among the plurality of the end points, such that the one or more further quantities are variable from each of the frames to the next in the sequence, and including allocating respective one or more further numbers of timeslots to be carried respectively to the one or more further end points.
Preferably, a period of each of the sequence of frames is substantially constant.
Preferably, each of the sequence of frames includes a header including respective window parameters for each of the plurality of end points, each window parameter including a time and a size of a window of upstream data which each of the plurality of end points is permitted to transmit to the central transmission point.
Further preferably, the respective window parameters are allocated by the central transmission point so that the windows do not collide at the central transmission point.
Preferably, the data includes at least a third quantity of third data for transmission to the first end point, such that the third quantity is variable from each of the frames to the next in the sequence, and including, responsive to the third quantity, allocating in each of the frames a third number of timeslots to carry the third data to the first end point, such that the third number is variable from each of the frames to the next in the sequence, and such that the first and third number of timeslots are not contiguous.
There is further provided, according to a preferred embodiment of the present invention, apparatus for downstream communication in a passive optic network by time division multiplexing of a sequence of frames, including:
a passive optical distribution fabric which is adapted to receive and convey data;
a plurality of receiving optical network terminations (ONTs) coupled to the fabric including first and second ONTs which are adapted to receive the data from the fabric; and
a central optical line terminal (OLT) coupled to the fabric which is adapted to receive the data for transmission into the network, the data including at least a first quantity of first data for transmission to the first ONT and a second quantity of second data for transmission to the second ONT, such that the first and second quantities are variable from each of the frames to the next in the sequence, to allocate in each of the frames a first number of timeslots to carry the first data to the first ONT and a second number of timeslots to carry the second data to the second ONT, such that the first and second numbers are variable from each of the frames to the next in the sequence responsive to variations in the first and second quantities of the data, and to transmit the data during the allocated timeslots.
Preferably, the data includes sets of data which are conveyed via respective different industry-standard services.
Preferably, the plurality of ONTs includes sets of ONTs operative at different wavelength groups, and the data includes respective sets of data which are conveyed between the OLT and the respective sets of ONTs via the different wavelength groups.
Further preferably, the data includes sets of data which are conveyed via respective different channels, wherein each of the channels transfers data via a service coupled to the OLT and at least one of the ONTs.
Further preferably, each of the channels is assigned a respective bandwidth, and allocating the first and the second numbers of timeslots includes allocating the first and the second numbers of timeslots responsive to the bandwidth of each of the channels.
Preferably, assigning each of the channels the respective bandwidth includes changing the respective bandwidth to a different bandwidth responsive to a request received by the OLT.
Preferably, the OLT includes a memory, and the first and second numbers of timeslots are allocated responsive to respective first and second data parameters stored in the memory.
Further preferably, a total of the first number and the second number of the timeslots is less than or equal to a bandwidth of each of the sequence of frames.
Preferably, the data includes one or more further quantities of data for transmission to respective one or more further ONTs among the plurality of ONTs, such that the one or more further quantities are variable from each of the frames to the next in the sequence, wherein the OLT is adapted to allocate respective one or more further numbers of timeslots to be carried respectively to the one or more further ONTs.
Preferably, a period of each of the sequence of frames is substantially constant.
Preferably, each of the sequence of frames includes a header including respective window parameters for each of the plurality of ONTs, each window parameter including a time and a size of a window of upstream data which each of the plurality of ONTs is permitted to transmit to the OLT.
Preferably, the respective window parameters are allocated by the OLT so that the windows do not collide at the OLT.
Further preferably, the data includes a third quantity of third data for transmission to the first end point, such that the third quantity is variable from each of the frames to the next in the sequence, and the OLT, responsive to the third quantity, is adapted to allocate in each of the frames a third number of timeslots to carry the third data to the first end point, such that the third number is variable from each of the frames to the next in the sequence, and such that the first and third number of timeslots are not contiguous.
There is further provided, according to a preferred embodiment of the present invention, a method for communication between a transmission point of a network and an end point of the network by time division multiplexing of a sequence of frames, each of which is divided into multiple timeslots, the method including:
receiving at the transmission point data for transmission to the end point, the data including at least a first quantity of first data for transmission to the end point and a second quantity of second data for transmission to the end point, such that the first and second quantities are variable from each of the frames to the next in the sequence;
responsive to the first and second quantities, allocating in each of the frames a first number of the timeslots to carry the first data to the end point, and a second number of the timeslots to carry the second data to the end point, such that the first and second numbers are variable from each of the frames to the next in the sequence responsive to variations in the first and second quantities of the data; and
transmitting the data from the transmission point to the end point during the allocated timeslots.
Preferably, the first data is conveyed via a first channel and the second data is conveyed via a second channel between the transmission point and the end point, wherein the first data is conveyed via a first service and the second data is conveyed via a second service, the first and second services being coupled to the transmission point and the end point and being external to the network.
Preferably, the data includes a third quantity of first data for transmission to the end point, such that the third quantity is variable from each of the frames to the next in the sequence, and including, responsive to the third quantity, allocating in each of the frames a third number of timeslots to carry the third quantity to the end point, such that the third number is variable from each of the frames to the next in the sequence, and such that the first and third number of timeslots are not contiguous.
There is further provided, according to a preferred embodiment of the present invention, apparatus for communication in a network by time division multiplexing of a sequence of frames, including:
a receiver coupled to the network which receives data from the network; and
a transmitter coupled to the network which is adapted to receive the data for transmission into the network, the data including at least a first quantity of first data for transmission to the receiver and a second quantity of second data for transmission to the receiver, such that the first and second quantities are variable from each of the frames to the next in the sequence, to allocate in each of the frames a first number of timeslots to carry the first data to the receiver and a second number of timeslots to carry the second data to the receiver, such that the first and second numbers are variable from each of the frames to the next in the sequence responsive to variations in the first and second quantities of the data, and to transmit the data during the allocated timeslots.
Preferably, the first data is conveyed via a first channel and the second data is conveyed via a second channel between the transmitter and the receiver, wherein the first data is conveyed via a first service and the second data is conveyed via a second service, the first and second services being coupled to the transmitter and to the receiver, and being external to the network.
Preferably, the data includes a third quantity of first data for transmission to the receiver, such that the third quantity is variable from each of the frames to the next in the sequence, and including, responsive to the third quantity, allocating in each of the frames a third number of timeslots to carry the third quantity to the receiver, such that the third number is variable from each of the frames to the next in the sequence, and such that the first and third number of timeslots are not contiguous.